1. Field of the Invention
The invention relates to a gap adjusting apparatus capable of judging whether a gap between a platen and a print head is set suitably for the thickness of a printing sheet.
2. Description of Related Art
Conventionally, in this kind of apparatus, when required to change a printing density of a character to be printed on the printing sheet, the gap between the platen and the print head is adjusted manually based on the operator's experience.
Further, when printing sheets having different thicknesses are printed in parallel, for example, in the case of a printing sheet that has a label portion thereon and, therefore, the label portion and the printing sheet portion have different thicknesses, the gap is determined by the experienced manual operation on the basis of an intermediate thickness between both paper thicknesses.
However, when the gap between the platen and the print head is adjusted by the experienced manual operation as described above, the gap is not always within an adequate range and is outside of the adequate range in many cases. In such cases, if the printing operation is executed when the gap is not adequately adjusted, it results in not only remarkable deterioration of the print head but also an increase in running costs due to frequent exchanges of the print heads. Also, the degraded quality of the printed characters sometimes forces a reprint from the beginning of the material.
Especially, in the case of a print head, that is a wire dot head which performs its printing operation through a print ribbon, there arise problems such as a jam due to damage to a print wire or the print ribbon.
Moreover, although an apparatus capable of performing automatic adjustment is disclosed in U.S. Pat. No. 4,990,004, it still requires manual adjustment for a label sheet and the like. In this case, if the operator forgets to make a mode change between automatic adjustment and manual adjustment, the printing operation is carried out with the gap not corrected.